Describe the rules to create a DFD with example.

Subject Software Engineering
NU Year Set: 2.(b) Marks: 6 Year: 2017

When creating data flow diagrams (DFD’s), there are certain rules which must be followed. This rules allow for the DFD to make sense and also to be easily understood. In this blog, I will go through the rules which must be followed and show practical examples of these rules.

1. All data flows must flow to or from a process

All flows of data must be either coming from or going to a process. External entities can not flow directly to each other. A data flow can not link a data store to an external entity. Data can not move between data stores without first being processed.

2. A Process must have at least one input flow and one output flow.

When a process has input flow but no output flow, it is called a “black hole”. When a process has output flows but no input flows, it is called a “miracle”.

3. The inputs to a process must be sufficient to produce output flows.

A “grey hole” is when the outputs of a process are greater then the sum of its inputs. For example, if a customers name and address is an input, their bank details cannot be an output, as the process doesn’t have enough information to produce it.

4. Processes must transform data.

When naming data flows, adjectives should be used which show how processing has changed the data flow.

5. Data Flows cannot cross each other.

The flows of data can not cross each other. To overcome this problem, data stores and entities can be duplicated. However, processes cannot be duplicated. Data flows must be unidirectional.

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